Semiconductor components are mounted onto a wiring board with solder material. For example, Sn-3Ag-0.5Cu (wt. %) having a melting point of 220° C. is commonly used for soldering semiconductor components such as IGBTs (Insulated Gate Bipolar Transistors) to a wiring board.
FIG. 7 is a schematic diagram of a semiconductor component mounted on a wiring board. When the semiconductor component 1 is mounted onto the wiring board 2, a solder bath dipping system is used to solder an external electrode 4 of the semiconductor component 1 to a wiring board electrode 5 by using solder material 3, for example Sn-3Ag-0.5Cu (wt. %) which has a melting point of 220° C. Here, since the solder material 3 is heated to a temperature in the range of 250 to 260° C. by the dipping system, the temperature in the semiconductor component 1 can reach a temperature in the range of 250 to 260° C. The semiconductor component 1 contains a semiconductor device 6 and an electrode 7 joined with a joining material 8. When the joining material 8 melts inside the semiconductor component 1, a short circuit, a break, or a change in electrical characteristics of the semiconductor component can occur and a defect can occur in an end product. The joining material 8 used inside the semiconductor component 1 therefore needs to have a higher melting temperature than the highest temperature in the semiconductor component 1 that can be reached when soldering in the dipping system.
An appropriate joining material that has a melting point higher than 260° C. and does not contain Pb may be one that contains 90 wt. % or more of Bi (hereinafter referred to as “a joining material containing Bi as main component”) (for example, Bi-2.5Ag (wt. %) with a melting point of 262° C. or Bi-0.8Cu (wt. %) with a melting point of 270° C.). While Zn is under investigation as another possible joining material, presently joining materials which contain Bi as main component are appropriate in terms of wetting characteristics and the ease with which a joint can be made. A power semiconductor module using a joining material which contains Bi as main component has been proposed (see Japanese Patent Laid-Open No. 2007-281412 (p. 24 and FIG. 2, for example)). FIG. 8 is a cross-sectional view of the conventional joint structure (power semiconductor module) described in Japanese Patent Laid-Open No. 2007-281412 (p. 24 and FIG. 2, for example).
In FIG. 8, a power semiconductor module 9 includes a Cu layer 13 formed on the surface of a power semiconductor device 10 on the joint 12 side for joining with a joining material. The Cu layer 13 is joined to an electrode 11 by the joint 12. The joint 12 is made of a joining material which contains Bi as main component.